Laser microfluidics: fluid actuation by light

TL;DR

This paper explores laser-based microfluidic manipulation techniques, including optical forces and thermocapillary effects, to enable contactless, reconfigurable fluid control in microchannels for advanced lab-on-chip applications.

Contribution

It introduces new methods for controlling two-phase microflows using optical radiation pressure and thermocapillary stresses, expanding the optical chip concept for microfluidics.

Findings

Optical radiation pressure can destabilize fluid interfaces and form liquid jets.

Thermocapillary stresses enable contactless manipulation of immiscible fluids.

Proposed optical toolbox includes valves, droplet sorters, and mergers.

Abstract

The development of microfluidic devices is still hindered by the lack of robust fundamental building blocks that constitute any fluidic system. An attractive approach is optical actuation because light field interaction is contactless and dynamically reconfigurable, and solutions have been anticipated through the use of optical forces to manipulate microparticles in flows. Following the concept of an 'optical chip' advanced from the optical actuation of suspensions, we propose in this survey new routes to extend this concept to microfluidic two-phase flows. First, we investigate the destabilization of fluid interfaces by the optical radiation pressure and the formation of liquid jets. We analyze the droplet shedding from the jet tip and the continuous transport in laser-sustained liquid channels. In the second part, we investigate a dissipative light-flow interaction mechanism consisting in heating locally two immiscible fluids to produce thermocapillary stresses along their interface. This opto-capillary coupling is implemented in adequate microchannel geometries to manipulate two-phase flows and propose a contactless optical toolbox including valves, droplet sorters and switches, droplet dividers or droplet mergers. Finally, we discuss radiation pressure and opto-capillary effects in the context of the 'optical chip' where flows, channels and operating functions would all be performed optically on the same device.